For example, power transmission devices of hybrid vehicles as described in Patent Document 1 to 4 exist. As described in Patent Document 1, such a power transmission device of a hybrid vehicle selectively transmits output of an engine, for example, an engine, and output of a rotator to drive wheels.
The power transmission device of a hybrid vehicle of Patent Document 1 includes an electric differential portion having a first rotating element, a second rotating element, and a third rotating element coupled to the engine, a first rotator, and a second rotator acting as the rotator, respectively, as depicted in FIG. 1 of Patent Document 1 such that the third rotating element, i.e., an output shaft, of the electric differential portion is coupled to drive wheels, that a rotor shaft of the second rotator is coupled via a meshing mechanism to a rotating member making up a portion of a power transmission path from the electric differential portion to the drive wheels, and that the output from the rotor shaft of the second rotator is transmitted to the drive wheels.
In the power transmission device of a hybrid vehicle as described above, when the output of the second rotator is absent or small, the rotor shaft of the second rotator enters a floating state and, if power including rotation variation from the engine is transmitted, teeth surfaces hit with each other between a meshing tooth of the rotor shaft and a meshing tooth meshed therewith on the rotation member side, and so-called tooth hitting noise or rattling noise may problematically occur.
In this regard, a power transmission device of a hybrid vehicle as described in Patent Document 5 exists. This power transmission device of a hybrid vehicle changes the engine rotation number to reduce tooth hitting if a condition of occurrence of tooth hitting of gears is detected in a drive train of the hybrid vehicle.